In-situ determination of bridging stresses in Al2O3/Al2O3-platelet composites by fluorescence spectroscopy

Bridging stresses arising from interlocking and frictional effects in the c rack wake have been quantitatively evaluated in an Al2O3/Al2O3-platelet cer amic, using in-situ microprobe fluorescence spectroscopy. Crack opening dis placement (COD) profile has also been quantitatively measured in the scanni ng electron microscope (SEM), in order to substantiate the reliability of t he piezo-spectroscopic measurements of microscopic bridging stresses. Mappi ng the crack wake (at critical condition for crack propagation) with a lase r probe of 2 mu m spatial resolution led to determine a discrete map of clo sure stresses over a crack extension of about 800 mu m. Relatively high bri dging stress values approximate to 350 MPa were revealed due to platelet in terlocking in a near-tip bridging zone less than 100 mu m, whereas friction al sites of lower stress magnitude less than 100 MPa were monitored in the crack profile farther away from the crack tip. The availability of microsco pic fracture parameters like as the bridging stress distribution and the ne ar-tip COD profile enables to quantitatively explain the rising R-curve beh avior of the Al2O3/Al2O3-platelet material. Bridging stress distribution, C OD profile and R-curve data are discussed in comparison with those collecte d in previous studies on equiaxed Al2O3 and roughened Si3N4 The present stu dy supports the notion that crack bridging is by far the most important tou ghening mechanism in non-transforming ceramics. (C) 1999 Elsevier Science L imited. All rights reserved.